namespace MaybePloeh

{

    using System;

    using System.Collections.Generic;

    using System.Linq;

​

    public class Program

    {

        public static void Main(string[] args)

        {

            var nonEmptyTitle = "The Old Man and the Sea";

            var nonEmptyAuthor = "Ernest Hemingway";

​

            var bookController = new BookController();

            Console.WriteLine(string.Format("empty title + empty author + GetBookFunctional = {0}", bookController.GetBookFunctional(string.Empty, string.Empty)));

            Console.WriteLine(string.Format("empty title + empty author + GetBookImperative = {0}", bookController.GetBookImperative(string.Empty, string.Empty)));

            Console.WriteLine(string.Format("empty title + empty author + GetBookImperativeSpecial = {0}", bookController.GetBookImperativeWithoutElses(string.Empty, string.Empty)));

​

            Console.WriteLine(string.Format("empty title + non empty author + GetBookFunctional = {0}", bookController.GetBookFunctional(string.Empty, nonEmptyAuthor)));

            Console.WriteLine(string.Format("empty title + non empty author + GetBookImperative = {0}", bookController.GetBookImperative(string.Empty, nonEmptyAuthor)));

            Console.WriteLine(string.Format("empty title + non empty author + GetBookImperativeSpecial = {0}", bookController.GetBookImperativeWithoutElses(string.Empty, nonEmptyAuthor)));

​

            Console.WriteLine(string.Format("non empty title + empty author + GetBookFunctional = {0}", bookController.GetBookFunctional(nonEmptyTitle, string.Empty)));

            Console.WriteLine(string.Format("non empty title + empty author + GetBookImperative = {0}", bookController.GetBookImperative(nonEmptyTitle, string.Empty)));

            Console.WriteLine(string.Format("non empty title + empty author + GetBookImperativeSpecial = {0}", bookController.GetBookImperativeWithoutElses(nonEmptyTitle, string.Empty)));

​

            Console.WriteLine(string.Format("non empty title + non empty author + GetBookFunctional = {0}", bookController.GetBookFunctional(nonEmptyTitle, nonEmptyAuthor)));

            Console.WriteLine(string.Format("non empty title + non empty author + GetBookImperative = {0}", bookController.GetBookImperative(nonEmptyTitle, nonEmptyAuthor)));

            Console.WriteLine(string.Format("non empty title + non empty author + GetBookImperativeSpecial = {0}", bookController.GetBookImperativeWithoutElses(nonEmptyTitle, nonEmptyAuthor)));

        }

    }

    internal class BookController : ApiController

    {

        /// <returns>

        /// 300 BadRequest with a list of causes for failure

        /// 200 Ok otherwise

        /// </returns>

        public IHttpActionResult GetBookFunctional(string title, string author)

        {

            var titleOption = NonEmptyString.Create(title);

            var authorOption = NonEmptyString.Create(author);

​

            var result = titleOption.Match(

                t => authorOption.Match<IHttpActionResult>(

                        a => new Ok(BookService.GetBook(t, a)),

                        () => new BadRequest("Invalid author")),

                () => authorOption.Match(

                        a => new BadRequest("Invalid title"),

                        () => new BadRequest(string.Format("Invalid title{0}Invalid author", Environment.NewLine))));

​

            return result;

        }

​

        /// <returns>

        /// 300 BadRequest with a list of causes for failure

        /// 200 Ok otherwise

        /// </returns>

        public IHttpActionResult GetBookImperative(string title, string author)

        {

            var titleOption = NonEmptyString.Create(title);

            var authorOption = NonEmptyString.Create(author);

​

            IHttpActionResult result;

            if (titleOption.IsSome)

                if (authorOption.IsSome)

                    result = new Ok(BookService.GetBook(titleOption.SingleOrDefault(), authorOption.SingleOrDefault()));

                else

                    result = new BadRequest("Invalid author");

            else

                if (authorOption.IsSome)

                result = new BadRequest("Invalid title");

            else

                result = new BadRequest(string.Format("Invalid title{0}Invalid author", Environment.NewLine));

​

            return result;

        }

​

        /// <summary>

        /// Due to the fact the Option/Maybe Match function is binary, i.e. you must have a return value

        /// for each case, this approach below (statement based) cannot be reproduced functionally (expression based).

        /// </summary>

        /// <returns>

        /// 300 BadRequest with a list of causes for failure

        /// 200 Ok otherwise

        /// </returns>

        public IHttpActionResult GetBookImperativeWithoutElses(string title, string author)

        {

            var titleOption = NonEmptyString.Create(title);

            var authorOption = NonEmptyString.Create(author);

​

            var reasons = new List<string>();

            if (titleOption.IsNone)

                reasons.Add("Invalid title");

​

            if (authorOption.IsNone)

                reasons.Add("Invalid author");

​

            IHttpActionResult result;

            if (reasons.Any())

                result = new BadRequest(string.Join(Environment.NewLine, reasons));

            else

                result = new Ok(new Book());

​

            return result;

        }

    }

​

    // ******************************************************************************************************************************

    // Below this is stuff that I faked to resemble the .NET WebApi, NonEmptyString and Option since importing their nuget into 

    // dotnetfiddle doesn't seem to work. Not very relevant to the discussion about Maybe<Semigroup>

    // ******************************************************************************************************************************

​

    internal class NonEmptyString

    {

        private string input;

​

        private NonEmptyString(string input)

        {

            this.input = input;

        }

​

        internal static Option<NonEmptyString> Create(string input)

        {

            if (string.IsNullOrWhiteSpace(input))

            {

                return new Option<NonEmptyString>();

            }

            else

            {

                return new Option<NonEmptyString>(new NonEmptyString(input));

            }

        }

    }

​

    internal class Option<T>

    {

        private T input;

​

        public Option()

        {

            IsSome = false;

            IsNone = true;

        }

​

        public Option(T input)

        {

            IsSome = true;

            IsNone = false;

            this.input = input;

        }

​

        public bool IsNone { get; internal set; }

        public bool IsSome { get; internal set; }

​

        internal TResult Match<TResult>(Func<T, TResult> p1, Func<TResult> p2)

        {

            if (IsSome)

                return p1(input);

            else

                return p2();

        }

​

        internal T SingleOrDefault()

        {

            if (this.IsNone)

            {

                throw new Exception();

            }

            else return this.input;

        }

    }

​

    internal class Ok : IHttpActionResult

    {

        private Book book;

​

        public Ok(Book book)

        {

            this.book = book;

        }

​

        public static implicit operator string(Ok ok)

        {

            return "Some book";

        }

    }

​

    internal class BadRequest : IHttpActionResult

    {

        private string v;

​

        public BadRequest(string v)

        {

            this.v = v;

        }

​

        public static implicit operator string(BadRequest badRequest)

        {

            return badRequest.v;

        }

    }

​

    internal class ApiController

    {

    }

​

    public interface IHttpActionResult

    {

    }

​

    internal class BookService

    {

        internal static Book GetBook(NonEmptyString title, NonEmptyString author)

        {

            return new Book();

        }

    }

​

    internal class Book

    {

    }

}